ClickHouse/dbms/src/Functions/finalizeAggregation.cpp

#include <Functions/IFunction.h>
#include <Functions/FunctionFactory.h>
#include <Functions/FunctionHelpers.h>
#include <DataTypes/DataTypeAggregateFunction.h>
#include <Columns/ColumnAggregateFunction.h>
#include <Common/typeid_cast.h>


namespace DB
{

namespace ErrorCodes
{
    extern const int ILLEGAL_COLUMN;
    extern const int ILLEGAL_TYPE_OF_ARGUMENT;
}


/** finalizeAggregation(agg_state) - get the result from the aggregation state.
  * Takes state of aggregate function. Returns result of aggregation (finalized state).
  */
class FunctionFinalizeAggregation : public IFunction
{
public:
    static constexpr auto name = "finalizeAggregation";
    static FunctionPtr create(const Context &)
    {
        return std::make_shared<FunctionFinalizeAggregation>();
    }

    String getName() const override
    {
        return name;
    }

    bool isStateful() const override
    {
        return true;
    }

    size_t getNumberOfArguments() const override
    {
        return 1;
    }

    bool useDefaultImplementationForConstants() const override { return true; }

    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
    {
        const DataTypeAggregateFunction * type = checkAndGetDataType<DataTypeAggregateFunction>(arguments[0].get());
        if (!type)
            throw Exception("Argument for function " + getName() + " must have type AggregateFunction - state of aggregate function.",
                ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);

        return type->getReturnType();
    }

    void executeImpl(Block & block, const ColumnNumbers & arguments, size_t result, size_t /*input_rows_count*/) override
    {
        const ColumnAggregateFunction * column_with_states
            = typeid_cast<const ColumnAggregateFunction *>(&*block.getByPosition(arguments.at(0)).column);
        if (!column_with_states)
            throw Exception("Illegal column " + block.getByPosition(arguments.at(0)).column->getName()
                    + " of first argument of function "
                    + getName(),
                ErrorCodes::ILLEGAL_COLUMN);

        block.getByPosition(result).column = column_with_states->convertToValues();
    }
};


void registerFunctionFinalizeAggregation(FunctionFactory & factory)
{
    factory.registerFunction<FunctionFinalizeAggregation>();
}

}